1. Field of the Invention
This invention relates generally to a process for the conversion of heavy hydrocarbon oils into light hydrocarbon oils and, more particularly, to a two-stage process for the conversion of heavy hydrocarbon oils into light hydrocarbon oils wherein, in the first stage, a heavy hydrocarbon oil is subjected to thermal cracking and, in the second stage, the product of the thermal cracking is subjected to a catalytic hydrotreatment.
2. Description of the Prior Art
The heavy hydrocarbon oils to which this invention is applicable include crude oils, reduced crude oils, vacuum residues, heavy oils extracted from tar sand, liquified coal oils and mixtures thereof. These hydrocarbon oils usually contain a large amount of components with a boiling point of 900.degree. F. or more such as asphaltenes, heavy metals, sulfur compounds, nitrogen compounds or the like. The term "asphaltene" used herein means a substance which is insoluble in normal heptane. Such heavy hydrocarbon oils exist abundantly in nature, and while they are considered as promising hydrocarbon resources, they are presently used merely for producing extremely low grade fuel oils or asphalt for pavement of roads. In view of the present political and economical situation facing energy crisis due to the depletion of high quality petroleum resources in the near future, it is strongly desired to develop effective processes capable of converting heavy hydrocarbon oils into more useful light hydrocarbon oils. The term "light hydrocarbon oils" used herein means oils having a boiling point of not higher than 900.degree. F.
U.S. Pat. No. 4,017,379 issued to Iida et al proposes a two-stage process for the conversion of a heavy hydrocarbon oil into light hydrocarbon oils, in which, in the first stage, the heavy hydrocarbon oil is subjected to thermal cracking at a temperature of 400.degree.-800.degree. C. and a pressure of 1-200 Kg/cm.sup.2 and, in the second stage, the product from the first stage as such is subjected to a catalytic hydrotreatment at a temperature of 370.degree.-480.degree. C. and a total pressure of 50-300 Kg/cm.sup.2. According to this process, a crude oil extracted from tar sand may be converted into light fractions with a yield as high as 98%. This process, however, has been found to involve a problem that the hydrotreated product is not stable so that a sludge is formed during transportation or storage. Therefore, when such a product is subjected to a further refining treatment, for example, the wall surfaces of the apparatus are apt to be fouled. In addition, the product oil has been found to be poor in compatibility. Thus, the oil tends to separate into layers during storage. Poor compatibility also makes it difficult to blend the oil product with other hydrocarbon oils.